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Back to the Question of How Many People Converted

I want to return to the question of how quickly the Christian church grew in the first four centuries.  This will be part of chapter 6 of my book on the Triumph of Christianity.   If you want a fuller background to what I say in this post and the one to follow, see my earlier musings on May 16 of this year, at https://ehrmanblog.org/how-many-christians-were-there/

In two posts I’m going to lay out what I think we can say both about how many people became Christian and at approximately what rate.  For those of you who are math whizzes, I would love for you to check my calculations to see if I’m making mistakes.  For everyone I would love to hear your comments on my claims and hypotheses.   This is a draft of that part of my chapter, with part two to come tomorrow.  As you will see, I begin in medias res.


As a result of these considerations, I want to suggest some minor tweaks in the way we understand the rate of Christian growth.   In doing so I want to emphasize that there could never have been anything like a steady rate.   Populations ebb and flow for all sorts of factors; for the growth of the Christian church there are all sorts of imponderables.   How quickly were Christians dying in relation to the rest of the population?  Did they have more births?  Were persecutions occasionally cutting into their numbers?  Or did the bravery of martyrs actually increase their numbers?

Some scholars, including Stark, have tried to take such matters into consideration, but in fact probably none of them is probably significant.   Christians died and were born at about the rate of others — despite elite Christian authors claiming that Christians never practiced abortion or infanticide.   These claims have been shown to be more propagandistic than factual.   And it was extremely difficult in antiquity to grow a population on the basis of natural processes: at best, according to the most convincing findings, populations could increase 1% a year by more births than deaths.  Nor, as we have seen, did improved Christian health care probably lead to better mortality rates.  Moreover, not very many people were actually martyred to decrease the Christian population significantly (as we will see in the next chapter), and there is almost no evidence that watching martyrdoms led large numbers to join the faith.

And so for our purposes we will not take into account special features of Christianity that one might unreflectively suppose had led to a difference.    I will base my calculations on two fixed numbers. …

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  1. John4
    John4  June 23, 2016

    Also, since you’re dealing in ballpark figures, I’d think you would want to use an order of magnitude for your 100 CE estimate: “something like 8400 Christians” or “something like 8000 Christians”, rather than the overly precise 8381.

    Again, just a thought for your consideration, wonderful Bart! 🙂

  2. Avatar
    doug  June 23, 2016

    I wonder if the initial burst of new Xians had anything to do with Xians telling people that the amazing Kingdom of God was right around the corner, and if you join now, all your problems will be solved forever! But after a few decades, the Kingdom that had been imminent but never came was no longer as much of a draw.

  3. Avatar
    Stephen  June 23, 2016

    In your analysis are you going to differentiate between Jewish Christians and Gentile Christians? As time went by wouldn’t the net population of Jewish believers in Jesus be steadily decreasing?

    • Bart
      Bart  June 24, 2016

      I”m going to argue that hte Jewish mission died out pretty quickly, that by the time of Paul most Christians in most places were converts from paganism.

      • Rick
        Rick  June 25, 2016

        The Bar Hochba revolt ending with Jews being tossed out of Jerusalem would have pretty well done away with the Jerusalem church and perhaps other Jewish churches that looked to it (as Jews look to Jerusalem) for identity.

        I do think you have hit on a powerful point: That there was terrible urgency in the early Christian message – because he was coming back soon as the true messiah to finish his messianic apocalyptic work.

      • Avatar
        Stephen  June 25, 2016

        Prof Ehrman

        Do you know Oskar Skarsaune and Reidar Hvalvik’s “Jewish Believers in Jesus: The Early Centuries”? If so do you recommend it?


  4. Pattycake1974
    Pattycake1974  June 23, 2016

    On the May 16th post, you gave a growth rate of 1.7% per year for 100 years. From 6M to 30M Christians. I’m guessing the growth rate depends on what starting point you use? If you start at the very beginning, with 20 Christians, then the growth rate is 5% per year instead of 1.7%. That’s how I’m understanding this.
    Someone also commented on the May post that plagues wiped out mass populations and may have contributed, somehow, to the growing interest in Christianity. I can’t remember if you said you’d be addressing that or not. It would be good to know in what way growth was affected by plagues.

  5. Avatar
    Luke9733  June 24, 2016

    This is off topic, but have you heard of, or read, the book “The Messiah before Jesus” by Israel Knohl? My understanding is the book argues that the community that produced the Dead Sea Scrolls had an understanding of a suffering Messiah.

    • Bart
      Bart  June 30, 2016

      Nope! But I’ve heard his thesis and looked at his evidence. It doesn’t indicate what he says it does.

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    exPCman  June 24, 2016

    A further complication – what about varying rates of growth in various locations? Wouldn’t it be nice if we had specific information about specific congregations in specific urban areas around the Empire! This leads me to wonder if there is any such thing as “the history of Christianity” – perhaps it ought to be “the histories of local Christian congregations in various locations” (???). Not that this would make things any easier, but much more impossible to achieve! Part of the back-ground (or ground) for these questions arise from my long-standing unease with saying “Christianity” (i.e. in the singular) rather than “Christianities” … and this makes me wonder about different growth rates in different areas (cities and provinces) and how much growth was based upon the differences among these various Christian groups about “faith and practice” – did some grow more rapidly because they have greater “appeal” to the local population? Just here “thinking out loud” … and thanks for your postings triggering my thinking! P.S. My unease about using “Christianity” in the singular (re. “THE Christian faith” or “THE … anything”) is reflected in a similar perception of talking about “Judaism,” which (again) seems to be a word that should always be used in the plural!

  7. Greg Matthews
    Greg Matthews  June 24, 2016

    Are you ever going to address the effects of the numerous plagues known from the first few centuries of the common era? They would have served to keep the growth in check.

    • Bart
      Bart  June 24, 2016

      I’ve debated it, but the reality is that hte plagues would have affected the pagans as much as the Christians, so probably wouldn’t affect their relative numbers (I do argue this out a bit because some scholars have maintained that Christians would have had higher survival rates because of better health care)

      • Greg Matthews
        Greg Matthews  June 24, 2016

        The relative numbers of pagans vs Christians isn’t the point. What is important is that there would have been fewer Christians available to convert people and there would have been fewer pagans available to be converted therefore the growth rate of Christianity would have slowed. Unless you want to take the position that only the ones who were not doing any conversions were the only ones dying.

        There wasn’t just one instance of plague there were two major plagues before 400AD and I’m sure there were a number of localized plagues. Egypt was hit especially hard and that was a spot in which there were a lot of Christians.

        • Bart
          Bart  June 26, 2016

          Yes, that’s part of the reason I’m saying that there would not have been anything like a steady growth over those centuries.

  8. Avatar
    flshrP  June 24, 2016

    “And so let’s simply pick a sensible rate of growth, and say that for the first forty years, up to the time when Paul wrote his last surviving letter, the church grew at a rate of 300%”

    300% per year? Per decade? What?
    Linear growth? Exponential growth? Other?
    Need to spell out the details of the growth model you are using.
    Suggest you include an appendix in which you lay out all of your math analysis.

    “After Paul’s death there was almost certainly a rapid decline.”
    Don’t understand the logic here. It sounds like you are saying that Paul is so important a factor in the growth of the Christian movement that his death was some kind of severe jolt to the conversion rate. Why would Paul’s death make any difference in communities like Alexandria in Egypt that he, apparently, never visited?

    • Bart
      Bart  June 24, 2016

      Sorry — I thought I was being clear. 300% per year for a decade. Of course I’m not saying it was *actually* linear. But there is not way to break it down year by year or month by month etc.

  9. Avatar
    Wilusa  June 24, 2016

    There surely must have been converts who later became disillusioned and left the faith; and also, children of converts who resisted their parents’ attempts to raise them as Christians. Are you taking their possible numbers into account?

    Other factors that have to be considered: the possible drop-offs when evangelists had to acknowledge that the “Kingdom” wasn’t coming soon, and when they finally gave up on it and substituted the idea of a blissful “Heaven.” (I still find it hard to understand how a movement could survive making such a radical change in its core teaching. I’m guessing it was possible because of the poor communications in that era: people who heard the “new” doctrine didn’t realize there’d been a different, earlier one.)

    • Bart
      Bart  June 24, 2016

      Yes, my rate of growth are calculated on net gain, after deaths, apostasies, and so on.

  10. Avatar
    Scott  June 24, 2016

    I would suspect that the field for initial rapid conversion was prepared not just by those who had heard Jesus preach and then returned to their cities but by the followers of John the Baptist who were prominent enough that John received mention by Josephus. I don’t know if John’s teachings had reached Apollos in Alexandria (Acts 18:24-25) but Apollos had learned of them in a remote enough area that Jesus’ and John’s teachings got mingled and garbled together until Priscilla and Aquila had a chance to straighten him out.

    [I know I have trotted this theory out before but it seems especially relevant to this topic]

  11. Avatar
    Dipsao  June 24, 2016

    What added fuel to the fire of Christian expansion in the early years was an eschatological expectation. Paul, in his early original letters, anticipated something earth shaking to happen. That element of fear has a universal attraction, even as we see it abused among politicians in our own time. After Paul’s death and the failure of an imminent apocalypse, the hysteria eventually leveled off, giving way to business as usual and the rise of the pastoral letters which promoted hierarchy, organization and the household code.

  12. Avatar
    RonaldTaska  June 24, 2016

    I think, as I think you have written previously, that you mean a growth rate of 300% per decade. You left out “per decade” in the next to last paragraph in today’s post. I come up with:
    2048 by 70 CE
    3277 by 80 CE
    5243 by 90 CE and
    8389 by 100 CE. Close enough I would think. Maybe the extra 8 are the ones who survived Noah’s flood.

  13. Avatar
    BrianUlrich  June 24, 2016

    Last month I was reading about the Taiping Rebellion in China, and learned that during the first few years of Hong Xiuquan’s preaching, his followers hit four digits. Shortly thereafter the numbers become confused with politics, and so the situation is less comparable to early Christianity.

    • Avatar
      Hormiga  June 26, 2016

      > Last month I was reading about the Taiping Rebellion in China

      I think that searching out historical examples of the growth of new sects/movements might give some useful insights, as I’m doubtful that simple exponential models are adequate. The sheer number of far-flung churches that had appeared by Paul’s time suggests to me that the seed population of proselytizers exploded very rapidly in the ’30s. Kind of like the inflationary period after the Big Bang.

      Perhaps looking at statistics for, e.g., the Mormons or even new political movements would be worth doing.

  14. Avatar
    Manuel  June 24, 2016

    You have mentioned several times that at around the time of Jesus’s death there were about 20 converts. Yet the gospels indicate several times that Jesus spoke to crowds of hundreds. Doesn’t that indicate that in fact there were a whole lot more than 20 people?

    • Bart
      Bart  June 24, 2016

      I”m talking about people who believed in his resurrection immediately afterward, not about people he preached to before his death.

  15. Avatar
    RonaldTaska  June 25, 2016

    I think another way to word it is that during its first 30 years, the number of Christians quadrupled every decade.

  16. Avatar
    Wilusa  June 25, 2016

    A day after reading this, I can’t get it out of my mind: your saying there was no reason to believe the rate of abortion and infanticide was different among Christians than among others.

    Was infanticide common in those days? Was it a substitute for abortion – going through the pregnancy while intending all along to kill the newborn?

    • Bart
      Bart  June 26, 2016

      Yes, “exposure” of unwanted children was a not uncommon practice.

  17. Avatar
    Kazibwe Edris  June 25, 2016

    off topic question

    As you go, preach this message: ‘The kingdom of heaven is near.’ 8Heal the sick, raise the dead, cleanse the lepers, drive out demons. Freely you have received; freely give. 9Do not carry any gold or silver or copper in your belts.…

    so why did they not use these powers to bring jesus back to life???

    • Bart
      Bart  June 26, 2016

      God did that for them, in Christian opinion.

      • Avatar
        Kazibwe Edris  June 26, 2016

        apologists such as james white argue “the flesh was not god”
        so the disciples did not have power to raise “jesus flesh” even though they went around raising other fleshes?
        god used human flesh/human disciples to raise human flesh ,but not gods flesh????

        • Bart
          Bart  June 27, 2016

          Really? OK then!

          • Avatar
            Kazibwe Edris  June 27, 2016

            seriously white does not believe that the flesh bit is god. so if the disciples had power to raise dead back to life, then they should have raised jesus’ dead flesh.

  18. Avatar
    Eric  June 27, 2016

    In population growth modelling, there is another complication to be aware of (not that your model need include this complication, only that it probably increases the necessary early growth rate, in reality).

    It is a variation on the “gamblers ruin” problem, the asymmetry of survival/growth.

    The empire was not a homogeneous pot but a collection of (sometimes relatively isolated) communities. So if there were 10 Christians in some Levantine town, there is a seed for growth. But if a small number of believers all die in a locality (plague, whatever), it is like that “branch” of growth never even existed, as zero seeds nothing.

    So in early going and as new geographies are proselytized, the growth rate needs to be even higher.

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